Three German ciphers unsolved since World War II are finally being cracked, helped by thousands of home computers.

The codes resisted the best efforts of the celebrated Allied cryptographers based at Bletchley Park during the war.

Now one has been solved by running code-breaking software on a "grid" of internet-linked home computers.

The complex ciphers were encoded in 1942 by a new version of the German Enigma machine, and led to regular hits on Allied vessels by German U-boats.

Allied experts initially failed to deal with the German adoption in 1942 of a complex new cipher system, brought in at the same time as a newly upgraded Enigma machine.

The advancement in German encryption techniques led to significant Allied losses in the North Atlantic throughout 1942.

Forced to submerge during attack. Depth charges. I am following the enemy

Kapt Hartwig Looks25 November 1942

The three unsolved Enigma intercepts were published in a cryptography journal in 1995 and have intrigued enthusiasts ever since.

Although assumed to have little historical significance, they are thought to be among just a handful of German naval ciphers in existence still to be decoded.

Exponential growth

The latest attempt to crack the codes was kick-started by Stefan Krah, a German-born violinist with an interest in cryptography and open-source software.

Mr Krah told the BBC News website that "basic human curiosity" had motivated him to crack the codes, but stressed the debt he owed to veteran codebreaking enthusiasts who have spent years researching Enigma.

A check against existing records confirmed that the message was sent by Kapitanleutnant Hartwig Looks, commander of the German navy's U264 submarine, on 25 November 1942.

Sophisticated

During the war, teams of codebreakers based at Bletchley Park, in the UK, scrambled to unravel German communications in an attempt both to undermine the German war machine and to save the lives of soldiers and seamen.

Frantic codebreaking work was carried out in plain-looking huts

Using early computers, Bletchley Park decoded thousands of intercepts in a knife-edge race to head off U-boat attacks.

German messages were encoded using the fearsome Enigma machine, which used a series of rotors, often augmented by a so-called "plugboard", to scramble transmissions not meant for Allied eyes.

Stefan Krah's computerised codebreaking software uses a combination of "brute force" and algorithmic attempts to get at the truth.

The combined approach increases the chances of stumbling across a match by recreating possible combinations of plugboard swaps while methodically working through combinations of rotor settings.

Proud milestone

Bletchley Park and its codebreakers have been immortalised on television, in film and in best-selling novels.

Now a museum, staff at the site are not attempting to close the book on World War II by solving any remaining ciphers. That they leave to the enthusiasts.

But a spokeswoman said that Bletchley Park followed the M4 project with interest, describing Mr Krah's work as a "great tribute" to the achievements of the wartime codebreakers.

Ralph Erskine, who submitted the original intercepts to the journal Cryptologia in December 1995, told the BBC News website that cracking the German codes after more than 63 years would be an important milestone for amateur cryptologists.

"I think there is more satisfaction for people engaged in the project to know that they have been able to do something that Bletchley Park couldn't do," he said.